In the semiconductor industry, a plasma processing system may be utilized to perform processing (e.g., etching or deposition) on wafers. In general, a wafer may be secured on an electrostatic chuck (ESC) by electrostatic forces for the processing. In order to ensure the wafer is stable during the processing, the electrostatic forces applied on the wafer by the positive and negative terminals of the ESC may need to be balanced by adjusting power supply. To adjust the power supply, the values of the positive load current supply to the positive terminal and the negative load current supply to the negative terminal may need to be calculated or measured.
FIG. 1A illustrates a schematic representation of an example plasma processing system 160. Plasma processing system 160 may include a chamber 169. Inside the chamber there may be an electrostatic chuck 164 for supporting wafers, such as wafer 162, during processing utilizing plasma 179. Plasma processing system 160 may have a DC power supply 166 to provide the clamping voltage 172 for securing wafer 162 on chuck 164. The DC power supply may be programmable, e.g., utilizing a bias control or sensor input 198 and/or a clamp'control 197, for setting a bias voltage 170 that defines a center of the clamping voltage 172, which may be a voltage with two endpoints, e.g., positive high voltage (+HV) 129 and negative high voltage (−HV) 130 (also illustrated in the example of FIG. 1B).
In order to keep the wafer 162 secured, or “clamped”, on the electrostatic chuck 164 it may be required to tune bias voltage 170 for a condition such that bias voltage 170 matches a plasma induced bias voltage 194 across chuck 164; plasma induced bias voltage 194 may not be directly measured/adjusted. The required condition may be equivalent to a condition that the value of the positive load current 181 applied to positive terminal 185 is substantially equal to the value of negative load current 182 applied to negative terminal 186. If the values of the load current are substantially different, electrostatic forces supplied by positive terminal 185 and negative terminal 186 may be substantially different, and wafer 162 may be tilted. As a result, the yield of plasma processing may be reduced.
In order to ensure the values of the load currents are substantially equal, the values may need to be measured, sampled and/or calculated.
FIG. 1B illustrates a schematic representation of a prior art arrangement including an isolation amplifier 132 for measuring the value of positive load current 181 illustrated in the example of FIG. 1A. A similar arrangement may be made for measuring the value of negative load current 182 illustrated in the example of FIG. 1A.
In the prior art arrangement, with reference to FIG. 1A-B, DC power supply 166 may supply positive high voltage 129, e.g., with a value of 2000 V, into plasma processing chamber 169 through RF filter 187 and positive terminal 185; RF filter 187 and positive terminal 185 may be represented by an equivalent resistor 107. The arrangement may include a sensing resistance 100 disposed between a first terminal 151 and a second terminal 152, first terminal 151 and second terminal 152 may be disposed between a terminal of DC power supply 166 with positive high voltage (+HV) 129 and equivalent resistor 107.
The arrangement may also include an instrumentation amplifier 102 electrically coupled with first terminal 151 and second terminal 152 through resistor 117 and resistor 118, respectively. Instrumentation 102 may be configured to sense the voltage between first terminal 151 and second terminal 152, i.e., ΔV 171, in order to determine the value of sensing current 103 utilizing the resistance value of sensing resistor 100.
In order for instrumentation amplifier 102 to sense ΔV 171, isolation amplifier 132 may be employed to shift the operating point (or operating range) of instrumentation amplifier 102 up, for example, to a range of about 2000V−15V˜ about 2000V+15V. Having a high operating point, instrumentation amplifier 102 may be able to sense ΔV 171.
Typical equipment for measuring voltage may only be able to measure voltages in a range of about −15V˜ about +15V. In order to obtain the value of a high voltage output 116 of instrumentation amplifier 102, isolation amplifier 132 may also be configured to shift output 116 to a low voltage referenced output 134 that is within a range of −15V˜+15V relative to ground level 136. Low voltage referenced output 134 may be measured utilizing typical voltage measurement equipment. Subsequently, low voltage referenced output 134 may be utilized to calculate output 116. In turn, output 116 may be utilized to determine the value of ΔV 171, which may be utilized to determine the value of sensing current 103. The value of positive load current 181 may be assumed to be equal to the value of sensing current 103 according to the prior art arrangement.
In general, isolation amplifier 132 may be very expensive. Further, the capability of isolation amplifier 132 may be substantially limited. Typically, isolation amplifier 132 may only be able to shift the operating point of instrumentation amplifier 102 up to about 2.5 kV. The limitation may be insufficient to satisfy requirements of some plasma processing systems, which may have a high voltage input of about 6 kV or even higher.
Instrumentation amplifier 102 also may have limitations. For example, instrumentation amplifier 102 typically may have a sensing range of only from −10V˜+10V. Given such limitations, instrumentation amplifier 102 may not be able to satisfy the requirements of some plasma processing systems.